In a conference talk on “Direct Optical Vorticity Probing”, 14th European Turbulence Conference, Sep. 1-4, 2013, Lyon, France, the inventors disclosed microcapsules for seeding a liquid fluid to track translational and rotational movements of the fluid. The microcapsules are transparent, neutrally buoyant, spherical, and with a glass mirror encapsulated inside. On average, the microcapsules have a diameter of 70 μm. The refraction index is 1.334, almost the same as the refraction index of water. No further details of the microcapsules, however, were given.
In another talk “Vorticity Measurements in Taylor-Couette Flows” presented on the 66th Annual Meeting of APS DFD, Nov. 24-26, 2013, Pittsburgh, USA, the inventors indicated that the microcapsules comprising the features as indicated above were prepared using a micro-fluidic device in that droplets of a first liquid phase including the micro-sized mirrors are dispensed into a flow of a second liquid phase. No further details were given.
M. B. Frish and W. W. Webb, “Direct Measurement of Vorticity by Optical Probe”, Journal of Fluid Mechanics 107, 173-200 (1981) measured the rotation rate of single micro-sized beads to obtain the vorticity of a fluid seeded with the beads. The micro-sized beads were made of a transparent material and encapsulated flat mirror discs.
For the purpose of tracking both translational and rotational movements of a fluid seeded with microcapsules including micro-mirrors, it is important that the microcapsules are small and spherical so that they directly follow the fluid, and that only their micro-mirrors reflect light used for determining the position and orientation of the microcapsules, i.e. that this light is not deflected by the material encapsulating the micro-mirrors.
EP 0 484 546 A1 discloses a microcapsule and a method of making the same. The microcapsule contains core substances enveloped by a capsule film obtained by coagulating fine colloidal particles with an electrolyte. The capsule film is formed, through the use of electrolyte, by coagulating the materials of the film which consist of fine inorganic and/or organic colloidal particles. The method comprises adding a substance to be encapsulated to a dispersion (hydrosol) of fine colloidal particles in which water is used as dispersion medium, dispersing said dispersion in an oil medium to form an emulsion, and coagulating the fine colloidal particles in said emulsion by using an electrolyte. In case the substance to be encapsulated is a water-soluble material, it may simply be mixed in the hydrosol. The substance to be encapsulated may be a dye, pigment, medicine, agricultural chemical, perfume, synthetic material, adhesive, enzyme, bacterial cell, etc.
Ingmar Polenz et al.: “Controlling the Morphology of Polyurea Microcapsules Using Microfluidics”, LANGMUIR, vol. 30, no. 44, Oct. 16, 2014, pages 13405-13410 discloses the use of microfluidics to continuously produce mono disperse polyurea microcapsules having either aqueous or non-aqueous cores. The microcapsule shells are formed by the reaction between an isocyanate, dissolved in oil, and an amine, dissolved in water, at the surface of oil-in-water or water-in-oil drops immediately as they are formed. Different microcapsule morphologies can be generated by using this approach. The thickness of the microcapsule shell increases with an increases in the amine solubility in the oil allowing for controlling the shell thickness in a range from tens of nanometers to several micrometers. These microcapsules are provided for applications requiring the encapsulation, delivery, and release of active materials, such as self-healing materials, catalysts, agricultural chemicals, textile chemicals, and chemicals used in paper manufacturing.
US 2012/0129742 A1 discloses microcapsules including a core containing one or more alkali metal borates, optionally hydrated, dispersed in one or more lubricating base oils of mineral, synthetic or natural origin, and a polymer shell.
US 2012/0003285 A1 discloses a method for manufacturing capsule series. The method includes separately conveying a first liquid solution containing a first material and a second liquid solution containing a liquid polyelectrolyte. A series of drops is formed at an outlet, each drop including a central core formed from the first solution and a peripheral film formed from the second solution. Each drop is immersed in a gelling solution containing a reagent capable of reacting with the polyelectrolyte of the film so as to form the gelled casing. The second solution contains at least one surfactant before the former contacts the first solution.
There still is a need of a method of producing spherical microcapsules including light reflecting solid integral particles and a plurality of spherical microcapsules for seeding an essentially transparent fluid to track movements of the fluid both in translational and rotational directions producible by the method, which ensure that the microcapsules follow the flow translation and rotation faithfully and which allow for precisely determining the position and the orientation of the spherical microcapsules using light reflected by their solid integral particles.